Vehicle door-mounted sign holders have long been known in the prior art. But, such prior art devices still suffer several problems.
One major problem is that prior art sign holders may be removed from the vehicle without having to open the door. This is a particular problem for those who have expensive signs displayed on their vehicles, and therefore do not want them easily removed by a passer-by, yet wish to have the signs easily transferable to other vehicles.
Those sign holders which will adjust to various sizes of vehicle doors are burdened with many small parts which must be moved or adjusted, and thereby increase the likelihood of mechanical faults or failure. The cost of maintaining a device with many moving parts would also be inherently greater than one with few total parts and fewer still which move.
Another problem, related to devices which have their adjustment mechanisms located on the outside of a vehicle door, is the effect of weather on those parts which must be adjusted. Dirt and corrosion can quickly make it difficult or impossible to turn adjustment screws or adjust other small moving parts--a problem which increases with a larger number of moving parts.